Fiber-optical pressure detector

ABSTRACT

An optical fiber disposed between a light source and a photodetector is subjected at a number of points, equispaced along its axis, to a transverse pressure causing a significant attenuation of the transmitted luminous radiation. The optimum spacing of the pressure points is a function of the radius of the fiber core and of the refractive indices of its core and its envelope. These pressure points are formed by turns of a substantially incompressible helix which is wound around the fiber and which may be constituted by an internal or external rib of a surrounding flexible sheath of similarly incompressible material. The pressure may be applied by a piezoelectrical transducer and may be modulated by an electrical signal to be picked up by the photodetector.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 450,809 filedDec. 17, 1982, now U.S. Pat. No. 4,572,950.

FIELD OF THE INVENTION

My present invention relates to a device for the detection of pressurewith the aid of a transluminated optical fiber.

BACKGROUND OF THE INVENTION

It is known that the bending of an optical fiber, juxtaposed with alight source at one end and confronted by a photodetector at theopposite end, attenuates the luminous radiation transmittedtherethrough.

It has been found, in fact, that the effect of transverse deformation asa means for varying the transmitted radiation can be optimized byapplying pressure to the fiber simultaneously at relatively offset,diametrically opposite points equispaced along its axis by a distancedetermined by its structural and optical parameters.

Reference in this connection may be made to a report based on studies byG. Zeidler, published at the Second European Colloquium on Fiber-OpticalTransmission held Sept. 27, 1976 in Paris.

That report dealt particularly with periodic curvatures introduced inthe fibers by a pair of jaws with relatively staggered teeth offset byhalf the recurrence period of these deformations. This recurrence periodis a function of the wavelength of the transmitted light.

Various assemblies of this nature serving for the measurement ofmechanical forces are disclosed in Swedish Pat. No. 410,521. Theutilization of deformation of a light-guiding structure in an opticalstrain gauge is the subject matter of my prior U.S. Pat. No. 4,163,397.

The conventional use of jaws with fixedly spaced teeth as a means forperiodically deforming an optical fiber has serious drawbacks. Thus,such a pair of jaws can be supplied only to a fiber section of limitedlength and several jaws of identical structure would have to be providedif two or more such sections were to be deformed simultaneously.Alternatively, as taught in the above-identified Swedish patent, thejaws can be widened to accommodate several turns of a fiber loopedtherearound. The high precision required in the manufacture of theirclosely spaced teeth makes these devices in either case ratherexpensive.

OBJECTS OF THE INVENTION

The general object of my present invention, therefore, is to provide animproved device for the detection of pressure by fiber-optical meanswhich is free of the aforementioned drawbacks.

A more particular object is to provide a highly sensitive instrument ofthis character for converting electrical or mechanical phenomena intoluminous signals transmissible over great distances.

Still another important object of the invention is to provide a devicewhich extends the principles of my above-mentioned copendingapplication.

SUMMARY OF THE INVENTION

A device according to my present invention comprises, in addition to anoptical fiber interposed between a light source and a photodetector asdiscussed above, pressure-transmitting means encompassing at least asection of that fiber and including a member of substantiallyincompressible material which is helicoidally wound about the fibersection in a multiplicity of equispaced turns.

U.S. Pat. No. 4,226,504 proposes a system for protecting an opticalfiber from mechanical stress with the aid of a soft shock-absorbingthread helically wound about the fiber within a tubular jacket. Therecommended spacing of the turns ranges between 1/4 and 12 inches, orabout 6 and 250 mm. The cushioning effect of the thread and the jacketprevents them from transmitting significant radial pressures to thefiber. In contradistinction thereto, the helicoidally wound member ofsubstantially incompressible material--e.g. a metallic wire--included ina device according to my invention has no shock-absorbing effect butfaithfully transmits such radial pressures with resulting deformation ofthe fibers at points spaced apart by half the pitch of the turns.

Advantageously, pursuant to a more particular feature of my invention,the device further includes a flexible sheath of substantiallyincompressible material enveloping the fiber--or at least the sectionthereof which is to be subjected to deformation--as part of thepressure-transmitting means. The helicoidal member, in fact, may beconstituted by an internal or external rib of such a protective sheath.

For optimum performance, of course, the pitch of the turns should be sochosen that the axial separation of the pressure points satisfies theperiodicity condition referred to above. As will become apparenthereinafter, the optimum separation is on the order of a fewmillimeters.

The optical fiber is preferably surrounded by the substantiallyincompressible filament in accordance with the present invention and canbe included in a mat or the like in which a bend inevitably will beimparted to the sensor as a result of the structure in which it isincorporated. I have found that the incorporation of the optical fibersurrounded by the filament in a helical pattern in such mat, or the useof fiber as a pressure sensor, where the orientation of the fiber makesa certain amount of bending of the fiber necessary, can be effected ifone maintains the radius of curvature R of the pressure sensor above aminimum radius of curvature at which static losses cease. Morespecificatlly when the cable is bent so that its radius of curvature isless than a predetermined minimum radius of curvature, static lossesincreases due to the tightening of the helic with decreasing radius ofcurvature of the bend.

Thus I have found that the minimum radius of curvature R which the cableor sensor can sustain can be defined by the equation ##EQU1## where p isthe pitch of the helix,

f is the diameter of the optical fiber,

g is the play between the optical fiber and the filament which may alsobe referred to as the radial width of the gap (mean) between thefilament and the optical fiber.

I have also found that, while a metal filament is effective as notedabove, when the optical fiber is of circular cross section, it is highlyadvantageous to utilize a polymeric filament as the helix. Indeed, whileone might think that practically any polymeric filament could beutilized effectively for this purpose, in point of fact, polymericmaterials such as nylons, which one might think would afford the bestresults, are less effective than polyesters which have reduced shrinkageupon heating.

It has been found to be advantageous, moreover, to fix the filament inplace with some kind of bonding agent, preferably a silicone rubber.

BRIEF DESCRIPTION OF THE DRAWING

The above and other features of my invention will now be described indetail with reference to the accompanying drawing in which:

FIG. 1 is a somewhat diagrammatic perspective view, with parts brokenaway, of a pressure detector embodying my invention;

FIG. 2 is a fragmentary perspective view of a modified pressure detectoraccording to my invention;

FIGS. 3, 4 and 5 are views similar to that of FIG. 2, illustratingfurther modifications;

FIGS. 6, 7 and 8 are graphs relating to the operation of my improvedpressure detector;

FIG. 9 is a diagrammatic plan view of an implement for determining theoptimum periodicity of the pressure points of an optical fiber used in adevice according to any of FIGS. 1-5;

FIG. 10 is a sectional elevational view of part of an apparatus usingthe device of FIG. 1;

FIG. 11 is a diagram illustrating the relationship involved in thediscussion of optical fiber bending;

FIG. 12 is a graph relating the excess loss in decibels with respect toradius of curvature for filament-helix pitches of one wavelength λ, orthree wavelengths 3λ, respectively.

SPECIFIC DESCRIPTION

FIG. 1 illustrates a pressure-detecting device according to my inventioncomprising a cable 1 which includes an optical fiber 2 with one endilluminated by a light source 3 and another end confronting aphotodetector 4. A preferably metallic, substantially incompressiblethread 5 is wound helically about fiber 2 within a flexible cylindricalsheath 6 also consisting of a substantially incompressible material,e.g. a metallic or resinous foil.

In operation, the cable 1 of FIG. 1 is placed between a pair of flatjaws constituted, for example, by a bed 10 and a pressure plate 11 asillustrated in FIG. 10. Bed 10 is part of a structure also including alid 9 separated therefrom by columns 12. The space between lid 9 andplate 11 is occupied by a pressure generator 13 such as a piezoelectrictransducer expanding and contracting vertically in response to analternating electrical signal applied to an input 14 thereof. Suchpiezoelectric transducers are available from the firm Physik Instrumente(PI), Waldbronn/Karlsruhe, German Federal Republic. An expansion ofblock 13, braced against lid 9, exerts pressure upon plate 11 withresulting relatively inverted deformation of fiber 2 at the zeniths andnadirs of the turns of helical member 5. This deformation, as discussedabove, will attenuate the light transmitted through the fiber fromsource 3 to photodetector 4 (FIG. 1).

The cable 1 of FIGS. 1 and 10, with its sheath 6 of circularcross-section, can be inserted between jaws 10 and 11 in any angularposition relative to its axis. The wire coil 5, however, is somewhatcompression-resistant so that this device is suitable only for thedetection of measurement of pressures above a certain minimum magnitude.A more highly pressure-sensitive cable 1a, partly illustrated in FIG. 2,comprises a wire 5a with flattened turns shrouded by a similarlyflattened sheath 6a of generally elliptical cross-section. The innerwidth of sheath 6a in the direction of its minor axis corresponds to thefiber diameter plus twice the wire diameter while its widthperpendicular thereto is considerably greater. Such a cable 1a can beclamped between the jaws 10 and 11 of the apparatus of FIG. 10 only intwo angular positions 180° apart. The resistance of the long legs of thewire loops to radial pressure, however, is considerably less than thatof the wire shown in FIG. 1.

On the other hand, the heightened deformability of the structure of FIG.2 prevents its use under elevated pressures. In order to remedy thisinconvenience I have shown in FIG. 3 a modification of that structureaccording to which a cable 1b comprises a wire 5b and a sheath 6b,similar to their counterparts in FIG. 2, along with two metal bars 7spacedly flanking the fiber 2 within the loops of the wire. Thethickness of the bars 7 is slightly less than the fiber diameter, beingso chosen as to limit the deformation of the fiber under maximumpressure. Up to that limit, however, wire 5b and sheath 6b are stillhighly sensitive to transverse pressures exerted for example by anapparatus such as that shown in FIG. 10.

In all instances, the sheath surrounding the fiber may also be made oftransparent material to enable visual detection of light escaping fromthe illuminated fiber. This has been illustrated in FIG. 4 where atransparent sheath 6c, forming part of a cable 1c, has the same tubularshape as sheath 6 of FIG. 1. FIG. 4 also shows that the metal wire ofthe preceding embodiments could be replaced by a helical or helicoidalrib 5c formed integral with the sheath on its inner surface. A suitablematerial for the sheath 6c and its ribs 5c may be a polyacrylate.

Moreover, it is not absolutely essential that the helicoidally woundmember be disposed within the sheath. Thus, as shown in FIG. 5, a cable1d otherwise similar to those of FIGS. 1-4 has a sheath 6d provided withan external helical rib 5d. The rib could again be integral with thesheath but could also be formed by a partial removal of a layer ofmetallic or other suitable material bonded to the outer surface ofsheath 6d which in this instance closely surrounds the optical fiber 2.When the cable 1d is used in the apparatus of FIG. 10, its jaws 10 and11 bear of course directly upon member 5d to deform the fiber 2 throughthe intermediary of sheath 6d.

Optical fibers can also be produced with a helically twisted plane ofpolarization. See, for example, an article by A. J. Barlow, J. J.Ramskov Hansen and D. N. Payne titled "Birefringence and polarizationmode-dispersion in spun single-mode fibres", published September 1981 inApplied Optics, Vol. 20, page 2962. In such a case the helicoidalpressure-transmitting member ought to be of the same pitch as andaligned with the twisted polarization plane of the fiber.

In the case of ordinary optical fibers, having a core of mean refractiveindex n_(c) and an outer zone or envelope of lower refractive indexn_(o), the critical periodicity of the deformations has a wavelength λgiven by Field's equation as

    λ=2πr/(2Δ).sup.1/2                         (1)

where r is the radius of the fiber core and

    2Δ=1-n.sub.o.sup.2 /n.sub.c.sup.2 ;                  (2)

thus, equation (1) can be rewritten ##EQU2##

The numerical aperture NA of the fiber is given by ##EQU3##

Thus, the refractive core index n_(c) can be calculated from adetermination of the envelope index n_(o) and the numerical aperture NA.

In a specific instance, a fiber with progressively varying refractiveindex available under the designation Corning No. 41 292 205 was used.The fiber had a core diameter of 60μ, an envelope diameter of 125μ and anumerical aperture of 0.201. Its attenuation, for light having awavelength of 820 nm, was 4.2 dB/km. The fiber was found to have anenvelope index n_(o) =1.458 and a core index n_(c) =1.472. Calculationyielded the values Δ=9.33·10⁻³ and λ=1.380 mm.

For an experimental determination of the critical period λ, two sectionsof that fiber were clamped between a pair of disks each provided on oneof its faces with ten closely spaced cylindrical steel pins parallel toone of its diameters; the pins on the two disks were relativelystaggered by half their diameter. FIG. 9 shows one such disk 7 togetherwith its pins 8 and a bent fiber 2 including an angle α with a diameterperpendicular to the pins. By changing the angle α, the spacing of thepoints of contact P between the fiber sections and the pins was varied.,for α=0 that spacing corresponded to the pin diameter. Tests performedwith three pairs of such disks, having respective pin diameters of 1.0,1.5 and 3.0 mm, yielded the graph of FIG. 6 in which attenuation (in dB)is plotted against the spacing of the pressure points P (in mm). Thegraph shows a principal peak at 1.346 mm which agrees rather well withthe calculated value of λ=1.380 mm. Another, smaller peak exists at 4.1mm corresponding to the third harmonic of the period, i.e. to 3λ. Theclamping pressure was approximately 250 g.

FIG. 7 shows the variation of attenuation, at the optimum spacing λ ofthe deformation points P, with clamping pressure (in g). The attenuationis measured for an effective fiber length (encompassing the sectionssubject to deformation) of 29.92 mm, given by 20λ with the arrangementof FIG. 9. The curve of FIG. 7 is nearly linear in a range of 100 to 300grams, with a slope of 0.0526 dB/g representing the pressure sensitivityof the device. At a clamping pressure of 300 g the specific attenuationwas 4.67 dB/cm, the calculated distortion of the fiber was 2.85μ and thecalculated stress was 16.5 kg/mm².

A series of tests performed with a cable of the structure shown in FIG.1, including a fiber of the type referred to, yielded results listed inthe following Table I. The helical member 5 was a piano wire of 0.1 mmdiameter. Tests Nos. 1 and 2 were performed on the same cable withdifferent pitch of the wire, showing the significantly increasedsensitivity when that pitch equals 2A--i.e. twice the value given inequation (3)--in accordance with test No. 2. Test No. 3 was done withouta sheath whereas tests Nos. 4 and 5 used sheaths different from thatemployed in the first two tests. The two numerical values standing nextto the sheath material represent outer and inner diameter. The last fourtests were all carried out with the optimum pitch of 2×1.35 mm.

                  TABLE I                                                         ______________________________________                                        Test  Periodicity                 Sensitivity                                 No.   (mm)       Sheath           (dB/5 kg)                                   ______________________________________                                        1     1.2        Novoplast 1.3/0.5 mm                                                                           1.65                                        2     1.35       Novoplast 1.3/0.5 mm                                                                           8.00                                        3     1.35       None             17.10                                       4     1.35       Teflon 1.2/0.5 mm                                                                              0.75                                        5     1.35       Braiding of copper and                                                                         7.10                                                         Plastosyn 1.8/1.0 mm                                         ______________________________________                                    

A further test performed on a cable of the type shown in FIG. 1, coiledtwice about a cylinder of 1 cm diameter, showed an attenuation of 0.37dB compared with 0.40 dB for its optical fiber coiled twice about thesame cylinder without wire 5 and sheath 6. This test shows that thecable structure reduces the losses introduced in an optical fiber bentaround arcs of small radius.

A cable corresponding to that used in test No. 3, but with elongatedwire loops (as shown in FIG. 2) having an eccentricity of about 3, wasalso tested and found to have a sensitivity of 11.1 dB/kg. This isapproximately three times the sensitivity measured in test No. 3. Whenthe cable was wound in the aforedescribed manner about a cylinder of 1cm diameter, the resulting attenuation was 0.20 dB. This indicates thatthe gain in sensitivity is not obtained at the expense of static lossesin bends of significant curvature.

Additional tests were carried out on an unsheathed structure of the kindshown in FIG. 3 whose bars 7 had a thickness of 0.1 mm and a width of0.5 mm. Member 5b was a steel wire of 0.15 mm diameter spot-welded ontoboth surfaces of bars 7. With a pitch again chosen according to theoptimum periodicity given by λ=1.35 mm, the maximum static sensitivityof this structure was found to be 0.03 dB/g which is comparable to thatobtained for a fiber clamped between toothed jaws (e.g. as shown in theabove-identified Swedish Pat. No. 410,521). The loss determined upon acoiling of the structure about a cylinder of 1 cm diameter, as describedabove, was as low as 0.098 dB.

When the same structure was placed in an apparatus of the typeillustrated in FIG. 10, depth of modulation (in %) was measured as afunction of the intensity of the transmitted light ranging from zero to100% of the maximum transmissible radiation. The applied excitationsignal had a peak-to-peak voltage difference of 20 V which, with apiezoelectric transducer having an expansion coefficient of 1μ per 100V, corresponded to a displacement of 0.2ρ. As shown in FIG. 8, a maximumdepth of modulation equal to about 12% was obtained with 40% lighttransmission. The signal/noise (S/N) ratios for different bandwidths ofthe applied signal are listed in the following Table II.

                  TABLE II                                                        ______________________________________                                        Bandwidth     S/N Ratio                                                       ______________________________________                                        1          Hz     2.9.10.sup.-3                                               10         Hz     1.2.10.sup.-3                                               5          KHz    5.4.10.sup.-2                                               ______________________________________                                    

By dividing the values of the S/N ratio into the aforementioneddisplacement of 0.2μ, obtains the minimum displacement necessary for theS/N ratio of 1. With a bandwidth of 1 Hz, according to the first row ofTable II, this minimum displacement is as low as 0.69 Å.

The depth of modulation varies substantially linearly with excitationvoltage up to a maximum of about 150 V corresponding to a displacementof 1.5μ. This shows that a device according to my invention is highlysuitable for utilization as an analog modulator.

As will be apparent from FIG. 11, there is a minimum radius of curvatureR to which the optical fiber 2 can be bent when it is wrapped with ahelical filament 5, e.g. a polyester with no excess loss resulting fromthat bending of the optical filament and this minimum radius ofcurvature is related to parameters of helical winding. As can be seenfrom FIG. 11, the gap width g between the filament 5 and the opticalfiber 2 can be varied, the pitch p of the helix can be varied, and thediameter of the fiber f can be varied to adjust the value R_(mm) inaccordance with the relationship ##EQU4## to suit the need for theapplication of the sensor to various mats or the like in which thesensor is bent.

As can be seen from the diagram of excess loss measured as a function ofradius of curvature (FIG. 12) the measured results are close to thosedetermined theoretically from this formula where the pitch isapproximately equal to one wavelength λ. There is deviation where thepitch is 3λ but this case is hardly of interest from a practicalviewpoint because the sensor is nine times more sensitive to bendingthan with a pitch of λ for the same gap between the fiber and thefilament wound around it.

In order to make a helix of the filament around the optical fiber withthe desired mean gap width and a pitch of, say λ=1.35 m the polyesterfilament can be wound around a microtube through the interior of whichthe optical fiber is passed axially. The filament is coiled on themicrotube with its turns in contact with one another, i.e. with a pitchcorresponding to the diameter of the filament. The diameter of themicrotube is so selected that it will define the desired play or gapwidth between the optical fiber and the helix.

If, for example, the coiling of the polyester filament and the microtubeis effected at a speed of 220 rpm, the optical fiber is drawn axiallythrough the tube at a speed of 220×1.35 mm per minute corresponding tothe pitch λ of the helix.

At each rotation, one turn of the helix will be transferred from themicrotube onto the optical fiber so that both the desired pitch of thehelix on the optical fiber and the desired gap width are formed withoutproducing micro-parasitic static microcurvatures.

Best results are obtained when the filament is a polyester which onlyshrinks, after annealing heat treatment of filament before it is appliedto the optical fiber, about 1% upon being heated to 100° C. Moregenerally, the filament should have a shrinkage of at most 1.5% whenheated from 20° C. to 100° C.

I claim:
 1. A device for the detection of pressure, comprising anoptical fiber of a diameter f having at least one curve of a radius ofcurvature R, a light source juxtaposed with one end with said opticalfiber for injecting a light beam into said optical fiber, aphotodetector positioned to receive the light beam at an opposite end ofsaid optical fiber so as to detect modification of said light beamrepresenting pressure applied to said optical fiber between said ends,and a filament of a substantially incompressible material coiled aroundsaid optical fiber in a helix having a pitch p and with a mean spacing gbetween the helix and the optical fiber, said radius of curvature Rbeing greater than a minimum defined by the relationship ##EQU5##
 2. Thedevice defined in claim 1 wherein the pitch p corresponds to onewavelength λ of said light beam.
 3. The device defined in claim 1wherein said helix is a circular helix and said fiber has a circularcross section.
 4. The device defined in claim 3 wherein said filament iscomposed of a polymeric material having a shrinkage of at most 1.5% whenits temperature is raised from 20° C. to 100° C.
 5. The device definedin claim 4 wherein said filament is a polyester.